Functions

Functions are values. Declarations are hoisted; expressions are created when execution reaches them. Arrow functions are compact and capture this, arguments, and super lexically.

Multiple forms

function factorial(n) {
  if (n < 0) throw new RangeError("n must be non-negative");
  return n <= 1 ? 1 : n * factorial(n - 1);
}

const double = value => value * 2;
const total = (...values) => values.reduce((sum, value) => sum + value, 0);

Default parameters activate for undefined, not null. Rest parameters collect real arrays. JavaScript passes values; object-reference values allow shared mutation. Use normal methods when dynamic receiver this matters, arrows for lexical callbacks, and higher-order functions for reusable policy. Deep recursion can overflow because tail-call optimization is not broadly available.